Catalyst systems for cationic polymerization of isobutylene (IB) or C4 streams containing isobutylene (IB) (e.g. Raffinate 1) that can produce olefinic polymers of Mn=500-3000 with a reactive vinylidene at the terminus (HR-PIB) are of high commercial value. Catalysts based on BF3 complexes with alcohols or ethers have been used commercially, but they generally require low temperature and highly purified feed (U.S. Pat. No. 7,411,104 B2). Lewis acid-Lewis base complexes of aluminum halides or alkyl aluminum halides and ethers have also been disclosed in a range of media and with a variety of co-initiators. The initiators are primarily alkyl halides, H2O, HCl or ROH (e.g. Macromolecules 2010, 43(13), pp 5503-5507, Polymer 2010, 51, pp 5960-5969).
Getting high monomer conversions and high vinylidene in an apolar medium (suitable for commercial scale-up) using a continuous process without elaborate feed clean-up has been elusive. Catalysts that work well in a polar medium such as dichloromethane, often do not work in an apolar saturated hydrocarbon medium (Macromolecules, 2012, 45, pp 3318-3325).
One of the advances highlighted recently is that ethers with one or more electron-withdrawing groups (e.g. bis-2-chloro-ethyl ether, CEE) were particularly useful in enabling alkyl aluminum dichloride to initiate cationic polymerization in the presence of t-butyl chloride as co-initiator giving a high yield of HR-PIB (U.S. Pat. No. 9,156,924 B2). In the absence of the electron withdrawing groups, dialkyl ethers inhibited polymerization in an apolar medium (Macromolecules, 2014, 47 (6), pp 1959-1965) either because the Lewis acid-Lewis base complexes were too strong (high binding energy) or the resulting t-butyl oxonium ions were too stable. This made the rate of polymerization too slow to be commercially viable.
Even with complexes of the appropriate binding energy, small amounts of polar impurities such as acetone or methanol have been found to impede or inhibit polymerization. For example, only 30 ppm of acetone slows down the polymerization of IB drastically in the presence of 2000 ppm of EtAlCl2.CEE complex. Other polar impurities that can inhibit IB polymerization include higher alcohols, ketones, ethers, acetonitrile and carboxylic acids such as propionic acid. The total amount of polar feed impurities in many commercial feedstocks can be from 5 to about 200 ppm.
Surprisingly, applicants have found that the presence of a small amount of water surprisingly ameliorates the negative effect on polymerizations carried out using alkylAlCl2. CEE as catalyst and alkyl halide as initiator caused by a range of polar impurities commonly present in IB, and IB-containing feed streams. The present method enables lower cost processes that can use a broader range of feedstocks without expensive feed clean-up equipment. Though water itself can normally act as co-initiator for the polymerization of IB (US 2016/0333123 A1), the presence of alkyl halide as co-initiator is necessary to control the reaction and get reasonable monomer conversions with impure feed streams. The amount of water is also critical. Typically when water is used as a co-initiator for IB polymerizations it is generally present at concentrations of 5-100, e.g 10-50 mM (US 2016/0333123 A1). However, in the presence of alkyl halide, this amount of water can cause a decrease in vinylidene end-group selectivity. On the other hand, too little water does not eliminate the inhibiting effect of polar impurities on polymerization completely, especially if the molar concentration of water is lower than the concentration of impurities in the feed stream.